1. Field of the Invention
The present invention relates to a shock absorbing type steering device to be employed for absorbing a shock which will attack a driver in the event of collision of vehicles, and a method of producing the same.
2. Description of the Related Art
There has been heretofore used a shock absorbing type steering device having a first column in a cylindrical shape and a second column in a cylindrical shape which is press fitted into an opening at one end of the first column, and adapted to absorb a shock in the event of collision of a driver with a steering wheel mounted on a steering shaft which is covered with the two columns, by relative movements of the two columns in an axial direction.
In the shock absorbing type steering device as described above, the shock will be absorbed by friction force acting between the two columns when the first column and the second column move in an axial direction relative to each other. Therefore, in case where such shock absorbing type steering devices are manufactured in mass production, variations in loads of press fitting the second column to the first column corresponding to the friction force between the two columns must be minimized, so that the shock can be appropriately absorbed. Such variations in the press fitting loads can be minimized by decreasing dimensional tolerance of an inner diameter of the first column or an outer diameter of the second column. However, in order to decrease such dimensional tolerance, it is necessary to accurately work with an inner periphery of the first column or an outer periphery of the second column, and working cost will be increased.
Under the circumstances, it is considered that the first column is provided with swaged parts which are arranged in a row in a circumferential direction of the column on a peripheral wall thereof, respectively at two positions spaced in an axial direction of the column, and thereafter, the second column is press fitted into an opening at one end of the first column so as to press the two columns to each other by way of the swaged parts (Patent document No. 1). Because it is easier to accurately swage so that the dimensional tolerance of swaging depth may be decreased, as compared with a case of accurately working with the inner periphery of the first column or the outer periphery of the second column, variations in the press fitting loads can be minimized at a low cost.
[Patent Document No. 1]
Japanese Patent Publication No. JP-A-10-203381
However, in the conventional structure in which the swaged parts have been formed as described above, it has been impossible to sufficiently decrease those variations in the press fitting loads. Specifically, in the conventional structure, there have been formed swaged parts 101a, 101b which are arranged in a row in a circumferential direction of the column, on a peripheral wall of a first column 101 made of steel, respectively at two positions spaced in an axial direction of the column, as shown in FIG. 6, and thereafter, a second column 102 made of steel has been press fitted into an opening 101A at one end of the first column 101. In this case, a diameter D of a circle defined by tip ends of the swaged parts 101a positioned close to the opening 101A has been made equal to a diameter D of a circle defined by tip ends of the swaged parts 101b positioned remote from the opening 101A.
FIG. 7 is a graph showing relation between strokes and press fitting loads of the second column 102 with respect to the first column 101 on occasion of press fitting in the conventional case, as results obtained in five samples. In this case, the swaged parts 101a, 101b were formed at eight positions in a circumferential direction, respectively at two positions spaced in an axial direction of the column. An inner diameter D1 of the first column 101 was 34.06 mm, an outer diameter D2 of the second column 102 was 33.115 mm, the diameter D of the circles defined by the tip ends of the swaged parts 101a, 101b was 31.82 mm, a thickness of each of the columns 101, 102 was 2.0 mm, an axial length L of each of the swaged parts 101a, 101b was 13.8 mm, a distance L1 from the opening 101A to the swaged parts 101a close to the opening was 2.6 mm, and a distance L2 from the opening 101A to the swaged parts 101b remote from the opening was 49.6 mm. The press fitting loads in the five samples have slightly increased when the swaged parts 101a at the position close to the opening 101A of the first column have been pressed onto the outer periphery of the second column 102, and have largely increased when the swaged parts 101b at the position remote from the opening 101A have been pressed onto the outer periphery of the second column 102. It has been recognized that when the swaged parts 101b at the position remote from the opening 101A have been pressed onto the outer periphery of the second column 102, variations in the press fitting loads have amounted to 300N at most. An object of the present invention is to provide a shock absorbing type steering device in which such variations in press fitting loads can be decreased, and a method of producing the same.